1. Field of the Invention
The invention relates to a synchronous demodulator for demodulating a carrier signal modulated with a useful signal, this demodulator comprising a filter circuit for separating the carrier signal, a limiter for generating an amplitude-limited reference signal from the carrier signal, a phase shifter for shifting the phase of the reference signal through a phase angle of essentially 90.degree. with respect to the carrier signal, and a multiplier for multiplicatively combining the modulated carrier signal with the reference signal.
2. Description of Related Art
A circuit arrangement for gaining sound signals from a television signal is known from German Patentschrift DE-PS No. 28 43 977, in which this television signal is applied via an amplifier to a filter circuit tuned to the IF picture carrier signal and is subsequently applied to a limiter amplifier for the purpose of gaining a reference signal for the demodulation. The filter circuit filters the IF picture carrier signal with a part of the neighboring signals transmitted as double sideband signals. The signals thus filtered are considerably limited in amplitude in the limiter amplifier in such a way that the amplitude modulation produced by the sidebands is substantially eliminated and a reference signal corresponding to the IF picture carrier signal is available at the output of the limiter amplifier, this reference signal being applied to a single sideband mixer stage. Moreover, the phase is shifted by 90.degree. in this signal path for the purpose of gaining the reference signal. The mixer stage also receives the signal comprising the picture IF signal, this signal being also applied to the amplifier and to the filter circuit. In the mixer stage, which may be part of a phase discriminator, the sound signal comprises in the picture IF signal is demodulated with the reference signal obtained at the output of the limiter amplifier.
In this circuit arrangement, the signals passed through the filter circuit 29 are limited to a range around the IF picture carrier signal. For this purpose the filter circuit may be constituted by a single circuit having a quality of 50 to 80. Provided that the amplitude distribution of the vestigial sidebands passed by the filter circuit is at least substantially symmetrically situated around the frequency of the IF picture carrier, the vestigial sidebands only affect an amplitude modulation of the IF picture carrier. In the case of an asymmetrical amplitude distribution, the IF picture carrier also has an interference phase modulation. While the (remaining) amplitude modulation is substantially eliminated by the limiter amplifier, the interference phase modulation remains unchanged and reaches the mixer stage in which it causes distortions in the demodulated signal.